Reduction of cross talk



Oct. 13, 1925.

l.. ESPENSCHIED nnnuc'lou oF cnoss TALK .Filed nec. 11. 1922 2 sheets-sheet 1 ATTORNEY oct. 13, 1925. 1,557,031

l.. EsPENscHlED REDUCTION OF CROSS TALK Filed De. 11, 1922 2 sheets-sheet 2.

y 10 1213;I 151605 15' 21 25j 261,? 50. J5;

'ATTORM 1 Patented Oct. 13I 1925.

UNITED LSTATES 1,557,037- PATENT oFFlcE.

LLOYD EsrENscHEn, or HOLLIS, NEW Yonx, Assrencm. rro Armeniensl TELEPHONE .AND TELEGRAPH COMPANY, A conronArroN or New Yonx.

REDUCTION OF CROSS TALK.

Appucaun mea December 11, 19,22. serial m. 606,231.

Toi all whom tmay concern:

Beit known that I, LLOYD Esrnxsomnn, residing at Hollis, in the county of Queens and State of New York, have invented certain Improvements in the Reduction of Cross Talk, of whiclr the following is a specification. p n

This invention relates to multiplex i-ransmission systems and more particularly to means for and methods of reducing crcsstalk interference between the channels of multiplex carrier systems extending over circuits paralleling each other upon the same pole line, or the like.

It is well lmown that in transmission circuits of all kinds and,x in particular, in telephone circuits, where the lines parallel each other, either by reason Iof being carried along the same pole line or by reason of bein included in the same cable, the telephonic or other signals transmitted over one line will induce corresponding currents in adjacent lines, thereby producing what is known as cross-talk. This has led to the introduction in telephone circuits of what are known as tra the purpose of these transpositions being. to balance out the induced cross-talk currents.

In the case of multiplex systems involving carrier currents upon which the telephonic or other signals are superposed, the problem` of reducing cross-talk becomes one of much greater magnitude than in the case of ordinary telephonie transmission. This r follows because 4not only are the currents induced greater in the case of the higher frequencies involved, but also the number of transpositions required is much greater and the accurate spacing of the transpositions and of the wires is much more important. To state the idea more concretely, it Jis a well known physical fact that tliecurrent inducedbetween two adjacent circuits' r increases with the frequency of the inducing currentr and, consequently, for two `given circuits paralleling eachother the crosstalk in the second circuit, due to signaling currents in Vthe first circuit, will be greater iny the case of carrier` currents than in the case of ordinary telephonic'currents. It becomes at once obvious, therefore, that the means used to reduce cross-talk in the case of carrier currents must be more elaborate mission. l

thann the case of ordinary/telephonie trans-k nspositions at suitable intervals,

4 tions'may be effective it is necessary that a large number of transpositions occur per wave length of the 4highest frequency .involved; that is, if We consider a section of line extending between two transposition points, the section of line must be very short as compared with the wave length 'of the signaling currents employed. It follows at once that the number of transpositions re quired in the case ofa carrier transmission circuit involving wave lengths very much shorter than those employed in ordinary telephonie practice will be quite large. Hence, to effectively transpose the lines a very considerable expense .is invblved.

Furthermore, the transposing of 'lie wires, in orderto be effective, must be done with a certain degree of mechanical accuracy. Unfortunately, for mechanica-l reasons, it 1s practically impossible to accurately transpose lines for frequencies of the order employed in carrier transmission.` l'lheftheory of transpositions require that, in a length of line, known as a transposition section, within which all the circuits are mutually tion of a transposition point does not ooincide with the place at which a. pole is spaced, an error will be introduced in the transposition scheme. These errors may be neglected in ordinary telephone practice,but in the case of carrier transmission, where the distance between successive transpositions is much shorter, the proportion of error` is so much greater that it becomes a very serious'factor. As. a result, even though the required number of transpositions are actually introduced, the expected results will not-be obtained in the case of carrier transmission.

In view of the im erfect results obtained by transposition an the large expense in-v volved in this method of overcoming crosstalk in carrier circuits, it is sought b the present invention to overcome cross-ta df- 'balanced the transposition points be spaced v iculties by a simpler method' and apparatus which may be employed either in conjunction with the transposition method or as a substitute ltherefor. An analysis of the conditions producing cross-talk shows that the greatest diiiiculty arises between channels transmitting in opposite directions on adjacent circuits. If we consider the conditions at the near end of an outgoing channel on one line and an incoming channel on an adjacent'line itcwill be apparent tlhat the outgoing currents are very large in amplitude,`while the incoming currents on the adjacent line have been greatly attenuated during transmission from a distant point. Consequently, the cross-talk produced by the outgoing currents of large magnitude will have considerable magnitude as compared with the attenuated incoming currents, and in some instances it is conceivable .that the cross-talk currents might be even greater in magnitude than the incoming currents received from a distant station. On the other hand, if we consider two channels transmitr ting in the same direction on two adjacent lines, whilethe cross-talk currents induced at the near -end in the adjacent channels will be as large as before, their magnitude will beA much smaller relative to the large signal currents transmitted in said channel. The ratio between the induced current-and the transmitted current underthese conditions will be substantially the same at any point along the two lines, assuming, of course, that attenuation conditions are similar in the two circuits. It follows at once that if cross-tall:

v between channels on adjacent lines transmitting in opposite directions can be avoided, the cross-talk induced in channels transmitting in the samedirection will, in some instances, be suciently small to be disregarded. If, however, the cross-talk is too large to be disregarded it may be reduced by a `line but, in addition, all of the 'bands trans'- mitted in one directionupon a'given line will lie in a different reuency range or" ranges from all of the ban s transmitted in the opposite direction upon an of the parallel lines. By usin'g suita le selecting means, such as band filters, it will then be possible to prevent the cross-talk currents induced by carrier currents transmitted in one direction from entering the terminal apparatus of channels transmitting in the oppof site direction.

It is, of course, recognized that the idea of using different frequency ranges for transmitting in o posite directions upon a given line is ol Applicants invention, however, goes a step further and insures that the frequency ranges employed in transmitting in one direction upon any of the parallel lines will be different from the frequency ranges employed in transmitting in the opposite direction upon all of the parallel lines. It further involves so .arranging matters that any employed frequency ranges which are common to two or more parallel lines will be used for transmission in the same direction.

It by no means follows from the fact that it was heretofore the practice in some cases to separate the frequency ranges employed in transmitting in opposite directions upon a single line, that the application of this old principle of a number of parallel lines would produce the results'obtained by ap licants invention. Various conditions mig t arise to prevent the attainment-of applicants results. If the frequency ranges employed for transmitting in opposite directions upon each of two parallel lines are -segregated from each other, the systems upon the two adjacent .lines might still be so arranged that the frequencies employed in the two cases would be diiferent, so that there would be an actual overlapping in frequency between the frequency ranges employed for certain channels transmitting in opposite directions over vthe adjacent circuits. Furthermore, even though the systems on the two parallel lines employed identical frequency ranges and an identical arrangement of channels, so far as frequencies 1are concerned, the frequency bands employed for transmission from east to west upon the one line might happen to be used for transmitting from West'to east upon the adjacent lines. Obviously, under this condition, the very worst crow-talk conditions would obtain. To apply a plicants invention to this situation would? therefore, involve reversing the d1- rection of transmission upon one of the lines. As a matte-r of fact, beforeapplicants invention no carrier systems have ever been installed upon parallel lines under such cir- 115 cumstances as to avoid the two conditions just referred to, for the necessity of complying with these requirements was not appreciated.

The invention, may now be more fully understood from the following description when read in connection with the accompanying drawing, Figures 1 and 2 of. which represent the conditions obtaining where two adjacent circuits are transposed under 125 different conditions, Fig. 3 of which illustrates the distribution of the cross-talk curents in a carrier system to which applicants invention has not been applied, Fig. 4 of which shows the distribution of Currents 130 upon two adjacent lines to which applicants invention has been applied, Figs. 5 and 6 of which show different methods of segregating the frequencies upon adjacent lines in accordance with applicants invention and Fig.'7 of which compares the conditions in adjacent circuits where the frequencies are segregated in accordance with applicants invention with the conditions obtaining when the segregation in the two circuits does not follow theprinciples of applicants invention. v

Before proceeding to a detailed exposition of applicants invention, the method of over-l coming cross-talk by transpositions will first be discussed.4 Referring to Fig. 1, let rus consider two parallel circuits vL1 and 'L2. Let us assume that the wave lengths employed in transmitting over these circuits are such that one trans osition'must occur in every section of the line having a length equivalent to that indicated between the points a and b. This transposition may be obtained by crossing the wires of the line L2, for example, midway between the points `a and b. 1f" the'current flowing over the line L1 is offthe magnitude and direction indicated by the heavy arrows adjacent the line L1 between the points a and b then the direction and magnitude of the induced current in the line L2 may be as represented by the arrows adjacent the conductors of the latter line. These arrows,

it will be observed, are in such a di` 'jtained. In the first place, the transposition must occur at the cross-arm and the cross.

arm may notl coincide with the theoretical point at which the transposition should be made. Where-the wave lengths employed are long, and the interval between trans'- position points is large compared with the interval between adjacent poles, as in' the case of ordinary telepho-ne transmission, the shifting of the transposition from one crossarm` to the cross-arm on the next adjacent pole will be, of small consequence since a relatively small difference in the lengths of the adjacent half sections of Fig. 1 would result. Injthe case of carrier transmission, however, where the wave lengths are not nearly so long, the interval between .transposition points must be so short that the location of the transposition at the crossar1n nearest the theoretical transposition point point occurs at o, somewhat distant from the midpoint of th section.

Another factor which may cause a further `variationf from theoretical conditions may arise from the fact that the two parallel lines are not equally spaced upon the cross,

arms at all points or a condition which is more apt to arise, the wires of the two lines sag by different amounts between the two l poles. This produces a condition which is represented in the case of the line L2 in Fig. 2, showing thetwo wires of the` line at the right of the point c closer together than the corresponding wires at the left of the transposition point c. If,v now, the heavy -arrows adjacent to the line L1 represent the magnitilde and direction of the current owing over said line in the transposition section y terminating at the points a and b,- the induced currents and their magnitudes may be 'as represented bythe arrows adjacent the conductors 4of the circuit L2. As indicated, the induced currents owing over the two wires to the left of the transposition point c are much larger than the induced currents flowing over theetwo wires to the right of the point c. While the direction of current flow is such that the currents induced to'the right of the transposition tend to neutralize vthe currents induced to "the left of the transposition, a resultant crosstalk current will actually occur, as indicated by the dotted arrows at the point a.

From the above'V discussion,\it ,will be ob Avious that the method of transposition alone will not be satisfactory for the reduction of cross-talk when applied toa multiplex carrier system. This follows/ from two facts ,the induced currents will in any case be larger because of the higher frequencies involved and the irregularities in the wire and transposition spacing will result in the reduction of thel c1'o'ss-talk-currents by a smaller percentage than in the case of ordinary telephone transmission.

.Let us now consider the distribution of thefcross-talk currents. f Referring to Fig. 3, twrrlines L,l and L2 are shown which may be multiplexed for carrier transmission by 'means of terminal carrier apparatus, indicated schematically at A and B, so that communication may take place simultaneously in a well known manner between the istat-ionsassocia-ted with a plurality of terminal lines." Let us now consider that a cur rent is flowing over the line L1 and that this current has a value of 25 arbitrary units,

asrepresentedy by the arrow at the point a,

which lis assumed to be adjacent the termi- 'the induction at the point a.

that the current will be reduced to one-fifth its value at the point a, so that it will have a value of five units, as represented by the heavy arrow at the point b. At the point 0, adjacent the terminal B, the current will be aga-in attenuated to one-fifth the value at point b, so that it will have a magnitude of one arbitrary unit, as represented by the heavy arrow at the point c. If, now, a current flowing in the line L1 induces a current a thousandth as great in the line L2, the current induced in the line L2 from the line L1 at the point a will have a value of .025 units. This induced current will in eiect be transmitted towards both terminals A and B, this condition being indicated by the upper arrows adjacent the line L2 at the point a. Consequently, a current having a value of .025 units will be transmitted towards the station A at the point a, due to The current of this value transmitted towards the point b is attenuated during transmission so that at the point' b it has a value of .005 units and at the point c it hasl a value of .001 units.

The current induced in the line L2 at the point b will have a value of one thousandth of the currentoiiowing over the line L1 at` that point. Consequently, the induced current in the line L2 at the point b will have a value 'of2005 and this current tends to be transmitted towards both stations A and B,

as indicated by "the arrows in the middle dotted line adjacent the line L2 at the polnt b. This currentis attenuated to a value of `.001 in being transmitted from the point b to the point a and is attenuated to the same value, .001 during transmission from' the point b to the point c. Similarly, the current transmitted over the line L1 at the point c has been attenuated tol the value of one unit and the resultant current induced inthe line L2 at this point will have a value of .001 units. The induced current tends to be transmitted towards 'both stations A and B. This current in being transmitted from c to a, is attenuated to the value v of .00004 units, but at the point c, as already stated, will have the value of .001 units.

It will be observed that all of the currents appearing at the point c, as a result of induction at the three different points along the line, a, b and c, have the same Value .001 and the total induced current appearing at the pointe, as a result of induction at the points a, b and c, will be .003 in the worst case when all three currents add directly. On the other hand, the total current appearing at the point a will have a value of .02604, (in the worstcase when all three currents add directly) which is almost ten times as great as the current at the point ,y 4c`4 The magnitude of the current at the :Point a is d u'e largely t0 the Current directly induced from the line L,l to the line L at said point and the ratio may be larger t an mitted over the line L2 in the same direction as that which has already been considered as being transmitted over the line L1, and this signalin current has the samel value as that transmitted over the line L1, it will arrive at the point c with a value of one unit, whereas the cross-talk current induced by the line L1, transmitting in the same direction, will' have a value of only .003 or, in other words, is only three-thousandths as great as the signaling current. On the other hand, if we consider a signalingcurrent transmitted over the line L, in the op osite direction, it will 'be apparent that this signaling current will arrive at the point a with a value of one unit, whereas, the cross-talk at that point induced from the line L1 will have a value of .02604 units, or, roughly, almost three-hundredths of the signaling current. t

From the above, it becomes at once aparentthat if wecan eliminate the current lnduced by a given channel on one line in a channel transmitting in the opposite direction over another' line, one of the greatest difficulties due to cross-talk will disappear, and we will only have to deal with the cross-talk currents of relatively small magnitude appearing in channels transmitting in the same direction.

This may belvdone by arranging the channels on adjacent circuits as 'indicated by the curves of Fig. 5. In this ligure, two circuits 1 and 2 areconsidered, each having three channels transmitting in each direction. If in the case of each circuit the channels transmittingin one direction have assigned to them frequencies of 6,000, 9,000 and 12,000 cycles respectively and those transmitting in the opposite direction have carrier frequencies of 15,000, 18,000 and 21,- 000 cycles respectively, it will be apparent that all the channels transmitting in the same direction over both lines may be grouped in one frequency range, and all of the channels transmitting in the opposite -direction on both lines may be grouped in another range. The carrier frequencies and the direction of transmission are indicated by the arrows in Fig. 5, The channels may channel tus.

quencies may be separated from each otherby means of a high-pass and low-pass filter, the characteristics o which are represented y the upper curves in the figure. It vwill be noted that the point of separation between the two groups is the same in the case of circuit-1 as in the case of circuit 2, and that all channels in both circuits below this point transmit in the same direction and all channels in both circuits above this point transmit in the opposite direction.

When these circuits are arranged as above described, the distribution of'the lcross-talk currents will be as indicated in Fig. 4. In this figure, the filters Qschematically indicated at F1 and F2, are the low-pass and high-pass filters, whose characteristics are shown in the curves of Fig. 5. The lines L1 and L2 at the terminals are divided into two branches, passing through the filters and suitable carrier apparatus schematically indicated. The' carrier apparatus is of a type `welllknown in the art. The apparatus associated with the filters F1 at the station A serves toV translate the low frequency signals from a plurality of terminal lines into l signals of carrier frequency which are superposed vupon and transmitted over the mu tiplexed lines. The carrier apparatus associated with the filters F, at the station A, on the other hand, serves to translate the received signals of carrier frequency into low frequency signals and to distribute the low frequency signals to the proper terminal lines. The carrier apparatus at the station B is the same as that at station A except that the apparatus associated with the filters F2 is the transmitting carrier apparatus and the apparatus associated with the filters F1 is the receiving appara- The arrows associated with the carrier fapparatus indicate the direction of transmlssion.

. With the carrier frequencies arranged as indicated by the curvesof Fig. 5, it will be apparent that the cross-talk components induced in the line L2 by the currents flowing from west toeast in the line L1 can only How over the circuit Lz from west to east, in other words, the cross-talk components tend ing to iiow in the opposite direction and into the receiving channel at station A will be suppressed by the filter F2 and only the cross-talk of small magnitude corresponding to that illustrated at the point e in Fig. 2 will be transmitted. Likewise, for signaling currents transmitted from east to west in line L1, only the cross-talk currents of small magnitude transmitted in the same direction will be effective, the larger crosstallr currents transmitted inthe opposite' direction being-3 suppressed by the filter F1 at the station In certain instances, the elimination of the near end cross-talk in the manner above described would be suiiicient, as the far end cross-talk may 'be small enough in magnitude to be neglected. If, however, the far end cross-talk assumes such proportions as to beobjectionable, the lines may be transposed in the usual manner to still further reduce the cross-talk. The number of transpositions required will, of course, be greater than in the case of an ordinary telephone circuit and the transpositions will necessarily involve more expense than in the Ycase of the ordinary telephone circuit. The mechanical inaccuracy of the transpositions, however, will be a less'serious matter where the frequencies are oilset as above described for the reason that a much lesser volume of cross-talk will remain to be taken care of. Consequently, the method of frequency separation when supplemented by trans ositions will ver eifectively take care o the cross-talk pro lem and will not involve the expense of mechanically accurate transpositions, which would be necessary if the difficultyfwere to be overcome by the method of transposition alone. a

Instead of offsetting the frequencies in the manner shown in Fig. l, the various channels in the several circuits involved may be arranged as indicated in Fig. 6, in which the carrier frequencies employed are staggered. In other ,Words frequency bands transmitted in one direction will be interspersed between those transmitted in the opposite direction. With this method it is necessary, of course, that thesame frequency ranges be transmitted .in the same direction over all of the circuits involved inA however, involves either staggering or grouping the frequencies of all of the circuits, and staggerin or grouping them 1n such a manner that a 1 of the channels on all of the lines transmitting in the same direction will lie in a frequency range or ranges different from all of the channels Qn all of the lines transmittingfin the opposite direction.

It must not be assumed that thisinvolvs merely the obvious application to a plurality of circuits on the same pole line of the well known principle of staggerin or grouping the carrier frequencieswhich ad previously been employed on single circuits. For exnels on one of the circuits might be reversed without departing fromf the ,principle of grouping the frequencies. This also would result in the most severe cross-talk conditions.

It might also hap en that a different set of frequencies woul be assigned tothe carrier channels on one circuit from the set of frequencies assigned to the carrier channels on the other circuit. This also would result in a departure from the conditions required in the practice of `applicants invention.

These factorsrare illustrated in Fig. 7. Referring to this figure, the frequency distribution of two circuits B and C is shown. The arrangement of circuit B is typical of a well known distribution of frequencies involving the grouping principle as it was known prior to applicants invention. Three two-way channels are involved, each transmitting in one direction on carrier frequencies of 6,000, 9,000 and 12,000 cycles and in the opposite direction on carrier freqluencies of 15,000, 18,000Y and 21,000 cycles.

ircuit B likewise is typical of another system of distribution of frequencies in accordance with the grouping arrangement as it was known prior to app lcants invention. This is a. system involving four two-way channels.. These channels transmit in one direction on frequencies of 6,666, 10,000, 13,333 andl 16,666 cycles and transmit in the opposite direction upon frequencies of 23,- 333 cycles, 26,666 cycles, 30,000 cycles and 33,333 cycles. It is'at once apparent that both circuits B and C involve the grouping principle. Yet it is equally evident that the channels of circuit B transmitting on 15,000 and 18,000 cycles and the channels of circuit C transmitting on 13 333 and 16,666 cycles will produce near en cross-talk, each upon the other. Likewise, itis obvious that if the direction of transmission in each of the channels of circuit B, for example be reversed, near end cross-talk will be produced between the 9,000, 12,000 and'21,000 cycle channels of circuit B and the channels of circuit C transmitting on 6,666 cycles, 10,000 cycles and 23,333 cycles.

In order to comply` with the principles of applicants inventlon, the frequency distribution of circuit B should be changed to that shown in connection with circuit B. .For example, the carrie;` frequencies for transmitting in one direction might be 12,000,

nels on both circuits .is substantial y the same so that no near end cross-talk will result if all of the channels on all of the lines below the point of segregation transmit in the same direction and all the channels on all of the lines above this point transmit in the opposite direction.

It would also be possible to produce this result by shifting all of the frequencies of ,the circuit C down in the frequency spectrum about 6,000 cycles so that the point'of segregation in the case of circuit C would coincide with that of circuit B. Bearing in mind that the frequency distributions of circuits B and C are typical of two types of circuits which have actually been used in practice, it will be apparent thauapplicants invention involves a distinct departure from a mereapplication of the principle of frequency separation to a plurality of circuits.

It will be obvious that the general prin-` ciple herein disclosed may be embodied in other and diiferent forms without departing from the spirit and scope of the invention as defined in the following claims.

What is claimed is:

1. In a transmission system in. which a plurality of channels transmit in each direction over each of a plurality of lines paralleling each other, the method which consists in filtering out of any channel transmitting in a given direction over any -line near end cross-talk induced from all channels transmitting in the opposite direction over an adjacent line.v

2. In a transmission system in which a plurality of channels transmit in each dirc tion over each of a plurality of lines paralleling each other, the method which'consists in assigning to' all of the channels of each line transmitting in one direction frequencies diiferent from the frequencies used in transmitting in the opposite direction over any of the lines, thereby substantially eliminating near end cross-talk.

3. In a transmission system in which a plurality of channels transmit in each direction over each of a plurality of lines paralleling each other, the method which consists in so arranging the frequency bands used for transmission in one direction on all of the lines that they will be for the most 1 alleling each other, the method which consists in so assigning frequencies to the various channels that all of the channels transmitting in one direction on allof the lines will lie above a predetermined frequency and all of the channels transmittingin the .opposite direction on all of the lines will' eliminating near end cross-talk. v

6. In a transmission system in which a plurality of channels transmit in each direction over each of a lurality of lines `paralleling each other, t e method which consists inr so choosing the carrier frequencies of the various channels that the transmis` sion vbands used for transmitting in one direction in each circuit will not overlap the transmission bands used for transmit-l ting in the opposite direction on the same circuit, the `vtransmission bands of the various lines being so related to each other that all` of the bands transmitting in one direction over all ofthe lines'will coincide and all of the bands transmitting in the opposite direction over all of thelines will coincide,

thereby substantially eliminating nearfendv cross-talk.

7. In a transmission system in-r which a plurality of channels'transmit in each direction over each ofa pluralityof lines paralleling eachother, the method which consists in so choosing the carrier frequencies of the various channels that any band transmitting in a given direction over any one of the lines will lie in a different range from the corresponding band transmitting in the op osite direction over the same l1ne, and

vv-i l also lie in a different range from all of the bands transmitting in the opposite direction on adjacent lines, thereby substantially eliminating near end cross-talk. i'

8. In a transmission system in which a plurality of channels transmit in each direction over each of a plurality of lines paralleling each other, the method' which consists in so choosing-the carrier frequencies of the various channels that all of the bands transmitting in the same direction on all of `the lines will lie in ranges of the frequency "spectrum differing from the ranges employed by any of the bands transmitting in the opposite direction )over any of the lines,

thereby substantially eliminating near end 05 cross-talk.

9. In a transmission system in which a plurality of channels transmit in each direction over each of a lurality of lines paralleling each other, t e method which consists in so choosing the carrier frequencies of the various channels that all of the bands transmittin in the same direction on all of the lines wi l lie in ranges of the frequency spectrum differing from the ranges em- 75.

ployed by any of the bands transmitting in the opposite direction over any of the lines, and filtering from each of the channels" cross-talk induced by any channel transmitting in the opposite direction on any line, 8l

thereby substantlally eliminating near end cross-talk.'J

\10. In a transmission system in which a.

4plurality of channelstransmit in each direction over each of a plurality of lines paralleling each other, the method which consists in filtering out of any channel transmitting in a given ydirection over any line near end. cross-talk induced from all channels transmitting in the opposite direction over an adjacent line, and in transposing the conductors lof adjacent lines to reduce far end cross-talk.

11. Ina transmission system in which a plurality of channels transmit in each direction over each of a plurality of lines par- Valleling each other, the method which consists in assignin to all of the channels of each line transmitting in one direction frequencies different from the frequencies used lo( in transmitting in the opposite directionl over any of the lines, thereby substantially eliminating near end cross-talk, and 1n transposing the conductors of adjacent lines to '105 reduceffar end crossltalk. y

12. In a transmission system in Which'a plurality of channels transmit in each direction over each of a plurality of lines paralleling each other, the method Which consists indso arranging the frequency bands used for transmission in one direction on all of the lines that they Willl be for the most part out of coincidence r,with any of the frequency bands transmitting in the opposite direction'on any of the lines, there- 115 by substantially eliminating near end cross- `talk, and in transposing/the conductors of adjacent lines to reduce far end cross-talk. 13.` In a transmission system in which a plurality of channels transmit in each diin one direction on all of the I will lie below said frequency, thereby substantially eliminating near end cross-talk, and in transposing the conductors of adjacent lines to reduce far end cross-talk.

1.4. In a transmission system in which a plurality of channels transmit in each directionoverl each of a plurality of lines paralleling each other, the method Which consists in so choosing the carrier frequencies of the various channels that the transmisson bands used for transmitting in one direction on all of the lines Will coincide and the transmission bands used for transmitting in the opposite direction on all of the lines will coincide, thereby substantially eliminating 4near end cross-talk, and in transposing the conductors of adjacent lines to reduce ar end cross-talk.

15. In a transmission system in which a plurality of channels transmit in eachrdirection over each of a plurality of lines paralleling each other, the method which consists in so choosing the carrier frequencies of the Various channels that the transmisson bands used for transmitting in one direction ongeach circuitwill not overlap the transmission bands used for transmitting inv the oppositev direction on the same circuit, the transmission bands of the various lines being so related to each other that all of the bands transmitting in one direction'o-Ver all of the lines Will coincide and all of the bands transmitting in the opposite direction over lall of the lines Will coincide, thereby substantially eliminating near end cross-talk, and in transposing the conductors lo" adjacent lines 'to reduce far end cross-talk. t

16. In a transmission system in which a phuality of channels transmit in each direction over each of a plurality of lines paralleling each other, the method which consists in/so choosing the carrier frequencies of the Various channels that anyband transmittingin a given direction over any one of the lines will lie in a different range from the corresponding band transmitting in the opposite direction over the same line, and will also lie in a different range from all of the bands transmitting in the opposite direction on adjacent lines, thereby substantially eliminatin near end cross-talk, and in transposing t e conductors of adjacent lines to reduce far end cross-talk.

17. In a transmission system in which a plurality of channels transmit in each direction over each of a plurality of lines paralleling each other, the method which consists in soty choosing the carrier frequencies ot the various channels that all of the bands transmitting in the same direction on all of the lines will lie in ranges of the frequency spectrum differing from the ranges employed by any of the bands transmitting in the opposite direction overany of the lines, thereby substantially eliminating near end cross-talk, and in transposing the conductors of adjacent lines to reduce far end cross-talk.

l8. In a transmission system in which a plurality of channels transmit in each direction oiver each of a plurality of lines paralleling each other, the method Which consists in so choosing the carrier frequencies of the various channels that all of the bands transmitting in the same direction on all of the lines will lie in ranges of the frequencyj spectrum differing from the ranges employed by any of the bands transmitting in the opposite direction over any of the lines, filtering from each of the channels cross-talk induced by `any channel transmittin in the opposite direction on any line, t ereby substantially elminating near end cross-talk, .and in transposing the conductors of adjacent lines to reduce far end cross-talk. v

In testimony whereof, I have signed my name to this specification this-7th day of December 1922.

LLOYD EsPnNscHIEn 

